A067800 Nonprime numbers k such that phi(k) > k/2.

1, 9, 15, 21, 25, 27, 33, 35, 39, 45, 49, 51, 55, 57, 63, 65, 69, 75, 77, 81, 85, 87, 91, 93, 95, 99, 111, 115, 117, 119, 121, 123, 125, 129, 133, 135, 141, 143, 145, 147, 153, 155, 159, 161, 169, 171, 175, 177, 183, 185, 187, 189, 201, 203, 205, 207, 209, 213, 215

Offset

1,2

Comments

Sequence is similar to A014076(n) giving odd nonprimes. Only 3 terms = 105, 165, 195 are not in the sequence among 59 terms < 210.

Cototient(m) > totient(m) equivalent to 2*phi(m) < m; the missing terms mentioned here seem to form A036798. - Labos Elemer, May 08 2003

The number 9075 is not in this sequence, is in A014076 and is not in A036798, which means that the missing terms mentioned here do not form A036798 (cf. A118700). - R. J. Mathar, Aug 08 2007

Links

Amiram Eldar, Table of n, a(n) for n = 1..10000

Mathematica

Select[Range[250], !PrimeQ[#]&&EulerPhi[#]>#/2&] (* Harvey P. Dale, Aug 29 2021 *)

Prog

(PARI) isok(k) = !isprime(k) && eulerphi(k) > k/2; \\ Amiram Eldar, May 08 2025

Crossrefs

Cf. A000010 (phi), A014076, A036798, A051953, A083254, A118700.

Sequence in context: A071904 A326586 A014076 * A155474 A100819 A046446

Adjacent sequences: A067797 A067798 A067799 * A067801 A067802 A067803

Keyword

nonn

Author

Benoit Cloitre, Feb 07 2002

Status

approved